Why does 'kukka' turn into 'kukan' but 'pöytä' into 'pöydän'? Struggling with consonant gradation.

I'm a false beginner who keeps freezing up when I try to construct basic sentences because I overthink consonant gradation. I can read Finnish news headlines just fine, but the moment I have to change the stem of a word, my brain locks up. How do you internalize these KPT changes without doing math in your head every time you want to say something?